Rebiya Kadeer, the exiled Uighur leader, today claimed that almost 10,000 Uighurs had "disappeared" during ethnic unrest in China's north-western region of Xinjiang earlier this month and called on the international community to launch an inquiry.

Speaking during a controversial visit to Japan, Kadeer said Chinese authorities had used undercover "snatch squads" to target Uighurs during clashes between Uighur and Han Chinese in the city of Urumqi on 5 July.

"Almost 10,000 people attending the protests in Urumqi disappeared in one night," Kadeer, president of the pro-independence World Uighur Congress, said. "Where did they go? If they died, where are their bodies? If they were detained, where are they being held?"

It was unclear where Kadeer got her numbers from.

The state news agency, Xinhua, said today that authorities in western China had arrested 253 more people suspected of being involved in the violence in Urumqi, in addition to the 1,434 detained earlier over suspected involvement in the 5 July riot. There are no details of the ethnicity of suspects.

The violence began after police attempted to break up a peaceful protest against the killings days earlier of two Uighur workers by Han Chinese colleagues at a factory in Guangdong, southern China.

At least 197 people died – including 137 Han and 46 Uighurs – and 1,800 others were injured in the worst ethnic violence China has seen for decades.

China has accused Kadeer, 62, of inciting the riots and warned Japan that her visit could damage bilateral ties.

China's ambassador to Japan, Cui Tiankai, denounced her as a criminal. "How would the people of Japan feel if a violent crime occurred in Japan and its mastermind is invited to visit a third country?" he said in an interview with the Kyodo news agency.

Japanese officialstoday attempted to play down the significance of Kadeer's visit. "As a government, we understand that [it] came about as the result of a private invitation, and we do not think it will have a negative impact on the Japan-China relationship," the chief cabinet secretary, Takeo Kawamura, told reporters.

China also lodged a strongly worded complaint with Australia, where Kadeer is due to give a nationally televised speech next month, following the screening of a documentary about her life at the Melbourne international film festival.

Kadeer, who lives in the US, said the Chinese authorities had turned the peaceful demonstration in Urumqi into a bloodbath to justify a crackdown on Uighurs, a Muslim people who refer to the region as East Turkestan.

"For Uighurs, taking part in a demonstration is akin to committing suicide, so they would not have demonstrated without a good reason," she said.

A block on internet access imposed in Xinjiang three weeks ago has been partially lifted, but residents are still unable to make overseas phone calls. Normal service is not expected to resume until after 1 October, the 60th anniversary of the founding of the People's Republic of China.

Kadeer said China's Uighur population, which accounts for less than half of Xinjiang's 20 million people, would have little cause to mark the anniversary.

"We have not enjoyed freedom for the past 60 years," she said. "There isn't a single Uighur family that hasn't suffered at the hands of the Chinese Communist party.

"East Turkestan is rich in resources, but we have not seen the economic benefits. We are not allowed to freely practise our religion. As Uighurs, we have nothing to celebrate."